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首页> 外文期刊>Journal of Asian earth sciences >Blueschist metamorphism and its tectonic implication of Late Paleozoic-Early Mesozoic metabasites in the melange zones, central Inner Mongolia, China
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Blueschist metamorphism and its tectonic implication of Late Paleozoic-Early Mesozoic metabasites in the melange zones, central Inner Mongolia, China

机译：内蒙古中部混杂带晚古生代-早中生代成岩作用的Blueschist变质作用及其构造意义

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摘要

Blueschists in central Inner Mongolia are distributed as layers and blocks in melanges including the southern zone in Ondor Sum area and the northern zone in Manghete and Naomuhunni areas. They have been attributed to the subduction of Early Paleozoic oceanic crust. Blueschists from Ondor Sum and Naomuhunni are characterized by occurrence of sodic amphibole coexisting with epidote, albite, chlorite, calcic amphi-bole (in Ondor Sum) and muscovite (in Naomuhunni). Blueschists in Manghete contain porphyroblastic albite with inclusions of garnet and epidote in a matrix dominated by calcic-sodic amphibole, epidote, chlorite, albite and muscovite. Phase equilibria modeling for three blueschist samples using pseudosection suggest that the Al~(M2) contents in sodic amphibole can be used as a good barometer in the limited assemblage involving sodic amphibole + actinolite + epidote + chlorite + albite + quartz under pressures ＜4-6 kbar, while this barometer is largely influenced by temperature and bulk Fe_2O_3 contents in the actinolite-absent assemblage sodic amphibole + epidote + chlorite + albite + quartz of higher pressure and the Al~(M2) contents are not pressure-controlled in the albite-absent assemblage sodic amphibole + epidote + chlorite + quartz under pressures ＞ 7-10 kbar. In the sodic amphibole-bearing assemblages, the Na~(M4) contents in sodic amphibole mainly decrease as temperature rises, being a potential thermometry. The calculated pseudo-sections constrain the P-T conditions of blueschists to be 3.2-4.2 kbar/355-415 ℃ in Ondor Sum, 8.2-9.0 kbar/455 ℃-495 ℃ in Manghete and 6.6-8.1 kbar/420-470 ℃ in Naomuhunni. These P-T estimates indicate a rather high geothermal gradient of 18-25 ℃/km for the blueschist metamorphism, being of intermediate PIT facies series. Available zircon U-Pb age data suggests that the protoliths of blueschists were formed later than Late Paleozoic-Early Mesozoic and metamorphosed soon afterwards. An alternative interpretation for the tectonic implication of blueschists in central Inner Mongolia is that they may be a new type attributed to closure of limited ocean basins and do not represent a tectonic regime occurred in conventional subduction setting.
机译：内蒙古中部的蓝片岩分布在混杂物中，呈层状和块状分布，包括Ondor Sum地区的南部地区和Manghete和Naomuhunni地区的北部地区。它们被归因于早古生代洋壳的俯冲。来自Ondor Sum和Naomuhunni的Blueschists的特征是发生了钠闪闪的角闪石，其与附子，钠长石，绿泥石，钙质两性（在Ondor Sum中）和白云母（在Naomuhunni中）共存。芒格特（Manghete）的Blueschists含有卟啉质钠长石，其基质为钙质闪石，闪石，绿泥石，钠长石和白云母。使用伪切片对三个蓝晶岩样品进行相平衡建模表明，在有限的组合中，包括钠闪石+阳起石+附子+亚氯酸盐+钠长石+石英在压力小于4-的有限组合中，钠闪石中的Al〜（M2）含量可以用作良好的晴雨表。 6 kbar，而该晴雨表很大程度上受温度和缺少阳起石组合的钠闪石+附子+亚氯酸盐+钠长石+更高压力的石英中Fe_2O_3含量的影响，且钠长石中Al〜（M2）的含量不受压力控制压力> 7-10 kbar的情况下，无组装的钠闪石+附子+亚氯酸盐+石英。在含钠闪石的组合中，钠闪石中的Na〜（M4）含量主要随温度升高而降低，这是一种潜在的测温法。计算得出的伪截面将蓝sch的PT条件限制为Ondor Sum为3.2-4.2 kbar / 355-415℃，Manghete为8.2-9.0 kbar / 455℃-495℃，Naomuhunni为6.6-8.1 kbar / 420-470℃ 。这些P-T估计值表明，blueschist变质作用的地热梯度较高，为18-25℃/ km，属于中等PIT相系列。可用的锆石U-Pb年龄数据表明，blueschist的原石的形成要晚于晚古生代-早中生代，然后很快就发生了变质。内蒙古中部蓝sch的构造含义的另一种解释是，它们可能是由于有限海盆封闭而形成的一种新型类型，并不代表常规俯冲环境中发生的构造制度。

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